Tuesday September 03, 2024, 5pm GMT+2

Rengefors, Karin¹

Population genomic analyses reveal that salinity and geographic isolation drive diversification in a free‑living protist

1. Aquatic Ecology Unit, Department of Biology, Lund University, Sweden.

Species diversity, distribution and delimitation is challenging in most protists including dinoflagellates. Moreover, because of their small size, cryptic life cycles, and large population sizes, our understanding of speciation in these organisms is very limited. We performed population genomic analyses on 153 strains isolated from eight populations of the recently radiated dinoflagellate genus Apocalathium, to explore the drivers and mechanisms of speciation processes. Species of Apocalathium (previously Peridinium and Scrippsiella) inhabit both freshwater and saline habitats, lakes and seas, and are found in cold temperate environments across the world. RAD sequencing analyses revealed that the populations were overall highly differentiated, but morphological similarity was not congruent with genetic similarity. While geographic isolation was to some extent coupled to genetic distance, this pattern was not consistent. Instead, we found evidence that the environment, specifically salinity, is a major factor in driving ecological speciation in Apocalathium. By coupling RAD sequencing and transcriptome analysis, we could determine that saline populations had unique in RAD-loci coupled to genes involved in osmoregulation, while freshwater populations appear to lack these. Our study highlights that adaptation to freshwater through loss of osmoregulatory genes may be an important speciation mechanism in free-living aquatic protists.